Pressure swing adsorption is an important gas separation process which is widely used in the process and manufacturing industries. Pressure swing adsorption (PSA) is used for recovering high-purity gas products from crude process gas streams, for example in hydrogen production, or as an alternative to hauled-in atmospheric gas products or onsite cryogenic air separation processes. The PSA process has been highly developed for the separation of a wide variety of gas mixtures including, for example, the separation of air to provide oxygen and nitrogen products. For smaller product volumes in air separation applications, PSA processes may use a single adsorbent bed and one or more gas storage tanks to provide a constant product flow as well as gas for repressurization and purge. At higher product volumes, multiple adsorbent beds operating in parallel with overlapping cycles are used to generate a constant product gas flow as well as provide gas for repressurization and purge.
Each adsorbent bed in a pressure swing adsorption (PSA) cycle proceeds through a sequence of steps beginning with a feed or adsorption step in which a pressurized feed gas mixture is passed through a bed of adsorbent which selectively adsorbs one or more of the components in the mixed feed gas. A product gas containing the desired component at acceptable purity is withdrawn from the bed until the adsorption step is terminated at a predetermined time.
After termination of the adsorption step, the pressure in the bed is reduced in one or more steps in which gas is transferred at decreasing pressure to one or more other beds to provide pressurization gas to those beds. Final depressurization typically is completed by withdrawing a waste gas in a final waste depressurization or blowdown step. The depressurized bed then is purged with product gas or transfer gas provided from other beds, thereby removing additional adsorbed components and void space gas from the bed.
Upon completion of the purge step, the bed is repressurized to an intermediate pressure by one or more pressurization steps in which gas is transferred from other beds, and the bed then is pressurized further to the feed pressure with feed and/or product gas. The steps are repeated in a cyclic manner.
The transfer of gas from a bed at decreasing pressure to another bed at increasing pressure is an important and highly-developed feature of many PSA cycles. In this bed-to-bed gas transfer process, gas which is below product quality, but which still contains a significant concentration of the final product component, is transferred from the product end of a bed to the product end of another bed. This important step significantly increases product recovery, but must be carefully controlled to meet the required product purity. Optionally, gas of lower quality can be transferred from the feed end of the bed to the feed end of another bed.
Further refinement in the bed-to-bed gas transfer process holds promise for needed improvements in product recovery and product purity, and also for increased productivity, in the PSA process. In particular, there is a need for improved control of gas flow within a bed undergoing gas withdrawal during the gas transfer process. This need is addressed by the present invention as described below and defined by the claims which follow.
An embodiment of the invention includes a pressure swing adsorption process for recovering a less readily adsorbable component from a feed gas mixture comprising at least one less readily adsorbable component and at least one more readily adsorbable component, which process comprises performing cyclic process steps in a plurality of adsorbent beds, each bed having a feed end and a product end and containing adsorbent material which selectively adsorbs the more readily adsorbable component, each bed proceeding in turn through cyclic process segments which include an adsorption-make product segment, a first gas transfer segment in which gas flows from a bed initially at a higher pressure into one or more other beds initially at a lower pressure or lower pressures, a regeneration segment, a second gas transfer segment in which gas flows into a bed initially at a lower pressure from one or more other beds initially at a higher pressure or higher pressures, and a final repressurization segment. The gas transfer segments include
(1) transferring gas from the product end of the bed to the product end of another bed; followed by
(2) withdrawing waste depressurization gas from the feed end of the bed while continuing to transfer gas from the product end of the bed to the product end of another bed;
(3) during either or both of (1) and (2), transferring gas from the feed end of the bed to the feed end or ends of the one or more other beds; and
(4) prior to (1), transferring gas from the product end of the bed at a higher pressure to the product end of another bed at a lower pressure.
The ratio of the volume of gas transferred from the product end of the bed during (1) to the volume of gas transferred from the product end of the bed during (2) may be between about 3 and about 20.
During (2), the ratio of the volume of waste depressurization gas withdrawn from the feed end of the bed to the volume of gas transferred from the product end of the bed may be between about 0.1 and about 0.6.
Another embodiment of the invention relates to a pressure swing adsorption process for recovering a less readily adsorbable component from a feed gas mixture comprising at least one less readily adsorbable component and at least one more readily adsorbable component, which process comprises performing cyclic process steps in a plurality of adsorbent beds, each bed having a feed end and a product end and containing adsorbent material which selectively adsorbs the more readily adsorbable component, each bed proceeding in turn through cyclic process segments which include an adsorption-make product segment, a first gas transfer segment in which gas flows from a bed initially at a higher pressure into one or more other beds initially at a lower pressure or lower pressures, a regeneration segment, a second gas transfer segment in which gas flows into a bed initially at a lower pressure from one or more other beds initially at a higher pressure or higher pressures, and a final repressurization segment. The gas transfer segments include
(1) transferring gas from the product end of the bed to the product end of another bed; followed by
(2) withdrawing waste depressurization gas from the feed end of the bed while continuing to transfer gas from the product end of the bed to the product end of another bed;
(3) during either or both of (1) and (2), transferring gas from a point intermediate the feed end and the product end of the bed to the feed end or ends of the one or more other beds; and
(4) prior to (1), an additional step of transferring gas from the product end of the bed at a higher pressure to the product end of another bed at a lower pressure.
The ratio of the volume of gas transferred from the product end of the bed during (1) to the volume of gas transferred from the product end of the bed during (2) may be between about 3 and about 20.
During (2), the ratio of the volume of waste depressurization gas withdrawn from the feed end of the bed to the volume of gas transferred from the product end of the bed may be between about 0.1 and about 0.6.
The invention also relates to a pressure swing adsorption process for recovering a less readily adsorbable component from a pressurized feed gas comprising at least one less readily adsorbable component and at least one more readily adsorbable component, which process comprises performing cyclic process steps in two parallel adsorbers, each adsorber having a feed end and a product end and containing adsorbent material which selectively adsorbs the more readily adsorbable component. The cyclic process steps include:
(a) providing a pressurized feed gas at superatmospheric pressure and introducing the pressurized feed gas into the feed end of a first adsorber, selectively adsorbing a portion of the more readily adsorbable component on the adsorptive material, and withdrawing from the product end of the first adsorber a product gas enriched in the less readily adsorbable component;
(b) depressurizing the first adsorber by (1) transferring gas from an outlet at the product end of the first adsorber into the product end of the second adsorber and (2) withdrawing gas through an additional outlet disposed at a distance from the product end of the first adsorber and transferring this gas into the feed end of the second adsorber;
(c) continuing to transfer gas from the outlet at the product end of the first adsorber into the product end of the second adsorber and from the additional outlet of the first adsorber into the feed end of the second adsorber while simultaneously withdrawing waste depressurization gas from the feed end of the first adsorber;
(d) terminating all transfer of gas from the first adsorber to the second adsorber while continuing to withdraw waste depressurization gas from the feed end of the first adsorber;
(e) introducing product gas into the product end of the first adsorber while continuing to withdraw waste depressurization gas from the feed end of the first adsorber;
(f) pressurizing the first adsorber by (1) transferring gas from an outlet at the product end of the second adsorber to the product end of the first adsorber and (2) withdrawing gas through an additional outlet disposed at a distance from the product end of the second adsorber and transferring this gas into the feed end of the first adsorber, wherein the second adsorber is initially at a higher pressure than the first adsorber;
(g) further pressurizing the first adsorber by transferring gas from the product end of the second adsorber to the product end of the first adsorber and from the additional outlet of the second adsorber to the feed end of the first adsorber, and withdrawing waste depressurization gas from the feed end of the second adsorber;
(h) terminating all transfer of gas from the second adsorber to the first adsorber and further pressurizing the first adsorber by one or more steps selected from the group consisting of introducing product gas into the product end thereof, introducing pressurized feed gas mixture into the feed end thereof, and introducing product gas into the product end thereof while also introducing pressurized feed gas into the feed end thereof; and
(i) repeating steps (a) through (h) in a cyclic manner.
The additional outlet of the first adsorber in steps (b) and (c) may be at the feed end of the first adsorber and the additional outlet of the second adsorber in steps (f) and (g) may be at the feed end of the second adsorber. Alternatively, the additional outlet of the first adsorber in steps (b) and (c) may be intermediate the feed and product ends of the first adsorber and the additional outlet of the second adsorber in steps (f) and (g) may be intermediate the feed and product ends of the second adsorber.
The process may further comprise, following step (a) and prior to step (b), transferring gas from the product end of the first adsorber into the product end of the second adsorber, wherein the second adsorber is at a lower pressure than the first adsorber; and following step (e) and prior to step (f), further pressurizing the first adsorber by transferring gas from the product end of the second adsorber to the product end of the first adsorber, wherein the second adsorber is at a higher pressure than the first adsorber.
The process may further comprise, following step (e) and prior to step (f), terminating the withdrawal of waste depressurization gas from the feed end of the first adsorber and continuing to introduce product gas into the product end of the first adsorber. Alternatively, the process may further comprise, following step (a), terminating the introducing of the pressurized feed gas into the feed end of the first adsorber while continuing withdrawing from the product end of the first adsorber a product gas enriched in the less readily adsorbable component.